It is known that a significant reduction in noise generation by axial flow fans can be achieved by providing uneven or unequal spacing between the fan blades particularly at their tip portions.
An example of such a fan design in the prior art is illustrated and described in U.S. Pat. No. 5,000,660 to Houten et al. In the Houten patent, the fan blades are skewed to provide for uniform spacing of the blades at the root portion and unequal spacing at tip potions. Skewing of the blades is accomplished by altering the curvature of the blades between root and tip portions in a plane perpendicular to the fan axis. Thus, a gradual curvature of a blade center line from root to tip has a significantly different skew than a relatively sharp curvature of a blade center line running from the mid point of the blade at its root to its tip mid point.
While the foregoing blade design is generally satisfactory, it nevertheless has disadvantages particularly in difficulty encountered in plastic molding processes associated with the production of the blades. Blades with a relatively sharp curvature of their center lines are obviously more difficult and, or expensive to mold than blades having gradually curved center lines. Further, a number of blade configurations each with different curvatures of their center lines also adds to cost of production.
It is the general object of the present invention to provide an improved axial flow impeller which exhibits equal spacing between blades at their root portions and unequal spacing between the blades at their tip portions, and which may be manufactured at economic advantage while retaining and even improving noise reduction characteristics.
In accordance with the present invention and in fulfillment of the foregoing object, an axial flow fan is provided with a hub and a plurality of generally radial blades extending from the hub periphery. Each blade comprises a root portion adjacent the hub and terminates in a tip portion spaced radially from the hub. The root portions of the blades are spaced evenly at their junctions with the hub while the tip portions thereof are substantially unevenly spaced circumaxially due to the blades being pivoted about their center lines through unequal arcs about points along the junctions between the hub and the blade root portions. Preferably, the mid points of the junctions between the hub and the root portion are used as pivot points for the arcuate placement of the blades. Any desired pattern of unequal blade tip spacing can thus be achieved without resort to unequal curvature of the blades and, in fact, without resort to any substantial change in the configuration of the blades. Only the slightest modification in blade shape at root and tip portions is necessary from blade to blade.